To control access to image data, a technique for encrypting or scrambling image data is used. With this technique, image data is encrypted in advance by using an encryption key, and only persons who have a decryption key corresponding to the encryption key can correctly play back the image data.
Image data has an enormous information amount. To efficiently transmit and store such image data, it is often encoded (compressed). To encode (compress) image data, for example, a technique called JPEG2000 standard which is standardized by ISO/IEC JTC 1/SC 29/WG1 is used. The above-described image data encryption is sometimes executed for image data which is encoded (compressed) by the JPEG2000 standard. With this processing, image data can efficiently be transmitted and stored. In addition, access to the image data can be controlled.
Especially, according to the encoding (compression) technique such as the JPEG2000 standard, image data can be hierarchically encoded (compressed) in accordance with the resolution, image quality, spatial domain, and components which construct pixels. When image data having a hierarchical structure is subjected to encryption processing corresponding to such hierarchical structure, access control corresponding to the hierarchical structure can be implemented.
For access control depending on, for example, resolution, low-resolution components are not encrypted, and only high-resolution components are encrypted. In this case, access control can be executed such that everybody can play back the low-resolution components, and only authorized users (decryption key holders) can correctly play back the high-resolution components.
However, when image data having a hierarchical structure undergoes encryption processing corresponding to the hierarchical structure, an apparatus having no decryption key cannot execute decryption. Since the image data is decrypted in the encrypted state, the image is played back in a scrambled state. That is, it is difficult to play back an image in a non-scrambled state by an apparatus having no decryption key (for example, when access control is executed depending on resolution, as described above, it is difficult to play back only low-resolution image data without playing back high-resolution image data).